Coating for apex seals of rotary engines and method of making

ABSTRACT

A rotary internal combustion engine is disclosed having a rotor housing presenting an internal epitrochoid surface electrolytically coated with a composite of nickel and silicon carbide or coated with tungsten carbide. A rotor piston is mounted for planetary movement within the rotor housing and carries a plurality of apex seal strips each comprised of a wearresistant portion and a supporting portion. The wear-resistant portion is constituted of a fused composite of generally equally hard particle types, one type comprising martensitic stainless steel and the other particle type comprising a nickel-based alloy. At least one of the particle types contains molybdenum disulfide or graphite. An intermediate coating of nickel aluminide may preferably be used therebetween. The supporting portion is constituted of a material having a lower specific gravity than the wear-resistant portion and a greater yield strength. The supporting portion may be preferably comprised of aluminum.

United States Patent Telang et al.

[451 Sept. 3, 1974 COATING FOR APEX SEALS OF ROTARY ENGINES AND METHOD OF MAKING [7 5] Inventors: Yeshwant P. Telang, Grosse Ile;

James C. Uy, Dearborn Heights, both of Mich.

[73] Assignee: Ford Motor Company, Dearborn,

Mich.

[22] Filed: July 5, 1973 [21] App]. No.: 376,860

[52] US. Cl. 418/178 [51] Int. Cl. F0lc 21/00 [58] Field of Search 418/178, 179; 417/DIG. 1; 308/DIG. 8; 29/l96.6, 197

[56] References Cited UNITED STATES PATENTS 2,348,975 4/1944 Herman 418/179 X 3,033,180 5/1962 Bentele 418/179 X 3,554,677 l/l97l Zapf et al. 418/178 3,705,818 12/1972 ,Grosseau 418/178 X 3,718,443 2/1973 Faulkner et al 29/l96.6

FOREIGN PATENTS OR APPLICATIONS 724,353 2/1959 Great Britain 418/178 Primary ExaminerC. J. Husar Assistant ExaminerLeonard Smith Attorney, Agent, or Firm-Keith L. Zerschling; Joseph W. Malleck ABSTRACT A rotary internal combustion engine is disclosed having a rotor housing presenting an internal epitrochoid surface electrolytically coated with a composite of nickel and silicon carbide or coated with tungsten carbide. A rotor piston is mounted for planetary movement within the rotor housing and carries a plurality of apex seal strips each comprised of a wear-resistant portion and a supporting portion. The wear-resistant portion is constituted of a fused composite of gener- 18 Claims, 5 Drawing Figures COATING FOR APEX SEALS OF ROTARY ENGINES AND METHOD OF MAKING BACKGROUND OF THE INVENTION Rotary internal combustion engines have been known for some time and various types of materials have been utilized for the seals typically carried by the triangular-shaped rotor, called apex seals. The seals have been preferentially formed by compacting metallic powders followed by a sintering operation. Powdered aluminum with carbon has been used and so has powdered iron-titanium carbide mixture. Still another is a commerical application of tool steel impregnated with graphite; yet another is hot pressed silicon nitride. Each of such materials have certain operating deficiencies. For example, the powder compacted metallics are objectionable because of lower than desired wear resistance; another objection in some cases is expense of fabrication. The most prominent operating deficiency is that which appears as chatter or a great number of lines that are grooved or gouged out on the rotor housing surface as a result of direct contact by the seal strip during a single revolution of the rotor. Several revolutions produce a series of such transverse grooves which together appear microscopically as rather deep valleys and peaks along a specific zone of the rotor housing, usually about l/6 of the entire periphery. The differential height between the valleys and grooves of a severely chattered housing can be in the range of 50 to 350 mils. With such convolutions in the surface, the ability of the rotor seals to maintain gas-tight sealing becomes extremely difficult: lower efficiency for the engine is the result and premature failure of some of the engine components is prompted. None of the commerical applications have used coated composites, as taught herein.

SUMMARY OF THE INVENTION The primary object of this invention is to provide an improved construction for seal elements carried by a rotor of an internal rotary combustion engine, the construction having a composition compatible with compositions utilized on the rotor housing and against which a sliding engagement continuously takes place during each revolution of the rotor. More particularly, the composition of the seal elements is constituted so that friction between the seal elements and the rotor housing is reduced to increase the seal life, the composite density of the seal is reduced to avoid chatter from operating use and to be less subject to inertia for better sealing.

Another object of this invention is to provide a rotor and seal combination which is constituted so that the material presenting a wear-resistant surface for the seal element has a controlled degree of porosity and surface smoothness, as well as enhanced high temperature wear resistance, all at a lower manufacturing cost.

Yet still another object of this invention is to provide an apex seal element for a rotary engine which is formed of a composite of materials, one material being selected as an outer wear-resistant coating or portion and is structured to be thin but adherent to a support- SUMMARY OF THE DRAWINGS FIG. 1 is a fragmentary section view of a rotary engine showing the interengagement between the apex seals and the rotor and side housings;

FIG. 2 is a sectional view taken substantially along line 2-2 of FIG. 1;

FIG. 3 is a micro photograph of the bond between the supporting portion of the seal element with the wearresistant portion thereof, and 7 FIG. 4 is a graphical illustration of the loading experienced by a typical apex seal travelling the schematic path shown in FIG. 5.

DETAILED DESCRIPTION With reference to FIGS. 1, 2 and 5, the rotary internal combustion engine has two side wall housings 10 and 11 each having spaced parallel side walls 10a and 11a respectively. A rotor housing 12 presents an inner peripheral wall 13 connecting said side walls. The peripheral wall 13 is epitrochoidally shaped. Essentially the wall 12 comprises two lobes 12a and 12b against which an eccentrically mounted rotor 14 may have the apices 14a thereof in continuous engagement. The apices 14a of the rotor sliding engage the wall 13 thereby dividing the space enclosed by the housings and forming three variable volume working chambers A, B, C. The peripheral wall 13 of the rotor housing is provided with an intake port 15, ignition means 16 such as a spark plug and an outlet port 17, whereby a complete four-stroke cycle can be performed within each working chamber.

For effective operation of the engine, the working chambers must be sealed. For this purpose, each rotor apex 14a carries an elongated slug or strip 18 loosely fitting within a slot 19 defined in each apex. Each strip and slot extends substantially transversely across the width of the rotor as well as the rotor housing surface. The seal strip is urged by gas pressure as well as an auxiliary mechanical force (spring 20) to engage the epitrochoid surface 13 of the rotor housing throughout its operation. To do so continuously, the seal element must be moved laterally to engage a side of slot 19 to create a seal. In one zone of the planetary movement side 19a must be engaged; in another zone of the planetary movement, the seal element must shift to the other side 19b for creating a seal. Gas pressure promotes such shift. Similarly gas pressure assisted by the spring force is used to urge the seal element outwardly to engage surface 13. Due to the gas urged seal quality, the seal element experiences inertial forces in certain zones of the epitrochoid path (usually not at the shift points) resulting in chatter. Some means must be provided to eliminate this phenomenon if seal efficiency is to be maintained.

This invention contemplates forming the seal strips 18 of a composite of materials, a supporting portion 22 being constituted of a material, such as aluminum KO-l, having a low specific gravity. Cast iron or titanium may also be used. Such materials have high strength and a low weight factor so that inertial effects are reduced. However, since such supporting compositions do not have the additional characteristics of controlled porosity and high temperature hardness stability, a wear-resistant coating or portion 23 is employed. Such wear-resistant portion 23 is formed from a sprayed on mixture of generally equally hard particle types, resulting in a composite consisting of chromium, nickel, iron, boron and silicon. One particle type comprises martensitic stainless steel, preferably type 420, and the other type comprising a nickel-based alloy. Each are defined as powders for admixture in a proportion of at least 3:1 and preferably 121. At least one of the powder types contains an addition of graphite or molybdenum disulfide.

The nickel based alloy comprises essentially at least 70 percent nickel, up to percent iron, 3-5 percent silicon, 2-4 percent boron. Chromium may be a constituent in the range of 10-17 percent. A preferred chemical analysis for the nickel-based alloy consists of 0.75 percent carbon, 13 percent chromium, 5 percent iron, 4 percent silicon, 3 percent boron and 75 percent nickel. When the preferred nickel-based alloy is mixed 1:1 with the preferred 420 type stainless steel, a resulting composite (applied according to the method to be described) comprises 0.5 percent carbon, 13.25 percent chromium, 1.5 percent boron, 2.5 percent silicon, 45 percent iron and 37 percent nickel. Such composite coating has a specific gravity of about 7.8 and a coefficient of thermal expansion of about 7.4 X lO /in- /in./F. for the temperature range of 100-1,200F. Average hardness is at least 30 R at a temperature of 700F. (average is indicated because hardness testing values will vary depending on what particles are engaged by the testing equipment). The composite chemical analysis should contain at least 40 percent iron and at least 35 percent nickel.

To provide a self-fused powder composite, particularly where the particle types are of generally equal high hardness, the presence of boron and silicon in one of the particle types is significant. Boron and silicon are present as dissolved constituents in the preferred nickel-based alloy and each have a lower melting temperature than the other elements of the particle type. Thus, during flame spray deposition of the admixed powders, the nickel-based particles have a melted surface substantially due to boron and silicon coming out as low melting eutectics. Under the impelling force of the spray gun, the particles are impacted on the coated surface so that the fluid surface of at least one of the particle types acts as a rigidizing network'to assist in cementing the particles together upon cooling.

The apex seals 18, and particularly the surfaces thereof (here coating portion 23) are subjected to thermal conditions considerably more stringent than other interengaging surfaces of a rotary engine. The seals undergo severe dynamic forces and constant rubbing causing unusually high continuous temperatures at the seal. Other portions of the rotary engine experience intermittent friction, but the seal surface experiences constant friction, causing substrate temperatures to average at least 700F. In addition, the seals undergo severe loading and dynamic forces. In FIG. 4, a graphical illustration depicts the calculated apex seal radial load in pounds as the seal moves about the complete epitrochoid surface 13 (see FIG. 5). Two very critical load conditions are experienced at 40 and 160 (taken with respect to the angle references in FIG. 5). The apex seal is rotating at 7,000 r.p.m. and each weighs about 1.5 grams. The spring applies a mechanical load of about 5 pounds and is neglected in the calculations.

The total radial load is comprised of a gas load and an inertia load. The gas load rises drasticallyat locations of 40 (spark plug firing behind seal movement).

The total load at these locations is respectively 250 pounds and 226 pounds. If the width of contact of the seal strip with the rotor housing is assumed to be typically 0.1 inch and the peak engine gas pressure is 500 psi, then the maximum seal pressure exerted can be about 728 psi, a very high pressure.

Accordingly the seal structure must withstand high loading and higher than usual temperatures. To this end, the admixed flame sprayed coating of this invention meets these needs admirably. Molybdenum disulfide or graphite was added equally to each of the powder types in the range of 2-5 percent to provide a metallurgical lubricity allowing the coating to withstand such rigorous conditions. Tests indicate the hardness of the coating does not fall off at temperatures of 700F. and maintains a minimum hardness of R 30. Such hardness stability, combined with the low-weight supporting portion 22, enables the seal to be affected less by the variances in loading leading to less chatter, better sealing efficiency and improved fuel economy for the engine.

In FIG. 3, a microphoto of a section of coating 23 and the seal is illustrated. The illustration lOOX magnification) depicts the thickness of G of the coating as adhered to the aluminum substrate F of the seal strip. The surface was etched with 10 percent nitric acid. The coating was flame sprayed to deposit spherical martensitic stainless steel particles A and elongated or impacted nickel-based alloy particles B. Randomly oriented pores or voids appear at C (as black areas) and the gray areas represent oxides or interstitial oxygen. An intermediate coating E of aluminum bronze was used to enhance the metallurgical bond between the aluminum and coating since the service environment will be very severe. Alternatively nickel aluminide may be used as an intermediate flame sprayed coating. The porosity of the coating 23 is controlled to be in the range of 3-9 percent and is effective to provide an oil film on the surface thereof to reduce friction without providing an undue viscous drag.

It has been discovered that the use of the wearresistant coating 23 operates compatibly against compositions or coatings 25 on the rotor housing selected from the group consisting of electroplated nickelsilicon carbide and tungsten carbide. Also the coating 23 surrounds a portion of the end of each strip 18 and compatibly operates against coatings 26 on the side housing selected from the group consisting of chromium plating and nitrided cast iron. This compatibility allows the seal strips to glide over the rotor housing with less interfriction resistance. The controlled porosity permits the oil, injected into the combustion chamber along with the fuel, to be absorbed in the pores and continuously supply and maintain a thin film of oil on the surface of the coating and between the rotor hous- A preferred method of applying, the wear-resistant coating 23 to the supporting portion 22, utilizes a flame spray technique wherein the admixed powders of stainless steel and nickel-based alloy are fed into an oxygen of acetylene torch for thermal deposition. Prior to deposition, the supporting structure of the seal strip is prepared. The slug or strip is machined to an undersize condition relative to its final configuration to accommodate the spray coating. To enhance bonding between the outer wear-resistant coating and the supporting structure, the strip is grit blasted to dimple the surface for coating and an intermediate coating of nickel aluminide is applied in the thickness range of 5-8 mils. The wear-resistant coating is then applied by flame spraying (after the strips have been heated to about 200F.) in the thickness range of -20 mils. The spray is directed so as to feather the thickness at the leading and trailing sides of the strip. After the spray coatings have cooled to ambient conditions, the outer wearresistant coating ,is machined to removed a portion thereof leaving a maximum thickness of about lO-l2 mils.

A preferred composite analysis for the entire deposited coating would be chromium 13.25 percent, boron 1.5 percent, nickel 37.0 percent and silicon 2.5 percent, carbon 0.5 percent and iron 45 percent. The hardness of the outer wear-resistant coating will be high but will vary, depending upon the mode of testing. Some of the individual particles in the deposit will have hardnesses as low as Rockwell C and as high as Rockwell C 60. However, the combination of hardness values allows the wear-resistant coating to be ground to a smooth finish, gives high wear resistance and controls the porosity for high oil retention.

We claim:

1. A seal element for use on a rotor subjected to planetary motion within a rotary engine, said element being subjected to increasing and decreasing radially directed inertial forces during each revolution of said rotor and said rotor carrying means for countering said inertial forces to continuously maintain engagement of said seal element with an outer housing, said seal element comprising:

a. a supporting strip of material having an elongated configuration effective to extend substantially transversely across said rotor at predetermined circumferential locations thereof, said supporting strip being formed from material having a specific gravity no greater than 7.5,

b. an outer wear-resistant coating comprising an alloy of chromium, nickel and boron held together by a metal binder, said alloy and binder being deposited by thermal spraying upon said supporting strip to a thickness in the range of l5-20 mils, said coating having a porosity in the range of 5-8 percent.

2. The seal element as in claim 1, in which an intermediate coating of nickel aluminide is sprayed upon the supporting body in a thickness range of 5-8 mils, and said outer wear-resistant coating is mechanically removed after deposition to result in a thickness range of 10-12 mils.

3. A rotatable body member and seal combination for use within a hollow outer body, said bodies defining a combustion chamber for a rotary engine, the combination comprising:

a. a rotatable body member presenting more than one apex for dividing the space of said chamber, each apex having a slot for carrying one of said seals,

b. a seal formed as an elongated strip fitting loosely within said slot and effective to be sequentially urged by gases within said combustion chamber against respective opposite sides of said slot during a complete revolution of the body member, said seal having an adherent wear-resistant portion, consisting of a self-fused composite of generally equally hard particle types, one type comprising martensitic stainless steel and the other particle type comprising a nickel-based alloy, at least one of the particle types having an addition of 2-5 percent by weight of an element selected from molybdenum disulfide or graphite.

4. The combination as in claim 3, in which the martensitic stainless steel is type 420.

5. The combination of claim 3, in which the composite chemical analysis of the wear-resistant portion contains at least 40 percent iron and 35 percent nickel.

6. The combination as in claim 3, in which the wearresistant portion has a porosity of 5-8 percent.

7. The combination as in claim 3, in which the strip supporting said wear-resistant portion is comprised of a material having a lower specific gravity and a higher yield strength than said wear-resistant portion.

8. The combination as in claim 3, in which said wearresistant portion has an arcuate face presented for engagement with the hollow outer body, said wearresistant portion being metallurgically bonded to said strip.

9. The combination as in claim 3, in which said nickel-based alloy further comprises about 5 percent silicon, about 3.0 boron and about 0.5 percent carbon.

10. The combination as in claim 9, in which said nickel-based alloy preferably is comprised of chromium of about 13 percent.

11. The combination as in claim 3, in which the supporting seal strip is comprised of a material having a specific gravity less than said wear-resistant portion.

12. The combination as in claim 3, in which said nickel-based alloy comprises about 5 percent iron, l-3 percent boron, and 1.5-3.5 percent silicon, the remainder nickel.

13. The combination as in claim 12, in which said nickel-based alloy contains l0-l7 percent chromium.

14. The combination as in claim 3, in which the hollow body has an end wall defining an epitrochoid and said rotatable body member is effective to' move through a planetary motion with said seals in continuous engagement with said epitrochoid surface, each of said seal strips extending generally parallel to the axis of rotation of said body member and substantially across the entire transverse extent of the hollow body, said seal strips being subjected to increasing and decreasing inertial forces during each revolution of said body member and subjected to a mechanical bias force to maintain engagement with said end wall, said wearresistant portion cooperating with the material of said end wall to increase the integrity and consistency of contact with said end wall.

15. The combination as in claim 14, in which the radial inertial forces acting on said seal strip exceed 200 pounds during eavh revolution of the rotatable member.

16. The combination as in claim 3, in which the sup- I porting seal strip is constituted of a material selected from the group consisting of cast iron, aluminum and plain carbon steel.

17. The combination as in claim 16, in which a coating of nickel aluminide is interposed between said wear-resistant portion and supporting strip.

18. The combination as in claim 16, in which the intermediate coating has a thickness between 5-8 mils and the wear-resistant portion has a thickness between 15-20 mils. 

1. A seal element for use on a rotor subjected to planetary motion within a rotary engine, said element being subjected to increasing and decreasing radially directed inertial forces during each revolution of said rotor and said rotor carrying means for countering said inertial forces to continuously maintain engagement of said seal element with an outer housing, said seal element comprising: a. a supporting strip of material having an elongated configuration effective to extend substantially transversely across said rotor at predetermined circumferential locations thereof, said supporting strip being formed from material having a specific gravity no greater than 7.5, b. an outer wear-resistant coating comprising an alloy of chromium, nickel and boron held together by a metal binder, said alloy and binder being deposited by thermal spraying upon said supporting strip to a thickness in the range of 15-20 mils, said coating having a porosity in the range of 5-8 percent.
 2. The seal element as in claim 1, in which an intermediate coating of nickel aluminide is sprayed upon the supporting body in a thickness range of 5-8 mils, and said outer wear-resistant coating is mechanically removed after deposition to result in a thickness range of 10-12 mils.
 3. A rotatable body member and seal combination for use within a hollow outer body, said bodies defining a combustion chamber for a rotary engine, the combination comprising: a. a rotatable body member presenting more than one apex for dividing the space of said chamber, each apex having a slot for carrying one of said seals, b. a seal formed as an elongated strip fitting loosely within said slot and effective to be sequentially urged by gases within said combustion chamber against respective opposite sides of said slot during a complete revolution of the body member, said seal having an adherent wear-resistant portion, consisting of a self-fused composite of generally equally hard particle types, one type comprising martensitic stainless steel and the other particle type comprising a nickel-based alloy, at least one of the particle types having an addition of 2-5 percent by weight of an element selected from molybdenum disulfide or graphite.
 4. The combination as in claim 3, in which the martensitic stainless steel is type
 420. 5. The combination of claim 3, in which the composite chemical analysis of the wear-resistant portion contains at least 40 percent iron and 35 percent nickel.
 6. The combination as in claim 3, in which the wear-resistant portion has a porosity of 5-8 percent.
 7. The combination as in claim 3, in which the strip supporting said wear-resistant portion is comprised of a material having a lower specific gravity and a higher yield strength than said wear-resistant portion.
 8. The combination as in claim 3, in which said wear-resistant portion has an arcuate face presented for engagement with the hollow outer body, said wear-resistant portion being metallurgically bonded to said strip.
 9. The combination as in claim 3, in which said nickel-basEd alloy further comprises about 5 percent silicon, about 3.0 boron and about 0.5 percent carbon.
 10. The combination as in claim 9, in which said nickel-based alloy preferably is comprised of chromium of about 13 percent.
 11. The combination as in claim 3, in which the supporting seal strip is comprised of a material having a specific gravity less than said wear-resistant portion.
 12. The combination as in claim 3, in which said nickel-based alloy comprises about 5 percent iron, 1-3 percent boron, and 1.5-3.5 percent silicon, the remainder nickel.
 13. The combination as in claim 12, in which said nickel-based alloy contains 10-17 percent chromium.
 14. The combination as in claim 3, in which the hollow body has an end wall defining an epitrochoid and said rotatable body member is effective to move through a planetary motion with said seals in continuous engagement with said epitrochoid surface, each of said seal strips extending generally parallel to the axis of rotation of said body member and substantially across the entire transverse extent of the hollow body, said seal strips being subjected to increasing and decreasing inertial forces during each revolution of said body member and subjected to a mechanical bias force to maintain engagement with said end wall, said wear-resistant portion cooperating with the material of said end wall to increase the integrity and consistency of contact with said end wall.
 15. The combination as in claim 14, in which the radial inertial forces acting on said seal strip exceed 200 pounds during eavh revolution of the rotatable member.
 16. The combination as in claim 3, in which the supporting seal strip is constituted of a material selected from the group consisting of cast iron, aluminum and plain carbon steel.
 17. The combination as in claim 16, in which a coating of nickel aluminide is interposed between said wear-resistant portion and supporting strip.
 18. The combination as in claim 16, in which the intermediate coating has a thickness between 5-8 mils and the wear-resistant portion has a thickness between 15-20 mils. 